The invention relates to a digital information transmission system using differential pulse code modulation and comprising an encoder in which the differential PCM signal to be transmitted is formed as the difference between the incoming linear PCM signal and a prediction signal obtained from a prediction filter comprising a local decoder receiving said differential PCM signal.
It is a known fact that in digital transmission networks normalized by the CEPT, the transmitted signals in each path are formed from 8-bit words which appear at a 8 kHz sampling rate, which produces a bit stream of 64 k bits/s in each path. Each 8-bit word results from compression in accordance with an approximate logarithmic law of each signal sample to be transmitted, coded linearly with 12 bits by means of pulse code modulation (PCM).
At present, endeavours are made to reduce in digital transmission networks, the bit stream in each path, with the aim of increasing the transmission capacity of these networks. A means to accomplish this, which is described in Italian Patent Specification No. 984,398 consists in deriving from the normalized PCM signal a differential PCM signal which is formed by means of the differential pulse code modulation technique and suitable for transmission with a reduced number of bits. In accordance with this technique, a differential digital PCM signal is obtained in an encoder by quantizing a difference signal between the linearized PCM signal and a prediction signal; this prediction signal is obtained from the output of a local decoder to which the differential PCM signal is applied, this signal being transmitted to the remote decoder. By using inter alia a syllabic compression system to match the size of the quantizing step of the difference signal to the level of this difference signal, it is theoretically possible to obtain a 4-bit differential PCM signal of a satisfactory quality which enables the transmission to the remote decoder with a bit stream of 32 k bits/s, thus doubling the capacity of a transmission path designed for a bit stream of 64 k bits/s.
However, the use of differential PCM signals for the transmission in existing networks presents problems which apparently have not yet been resolved. In the junction centres of the network, switching is to be effected on the compressed PCM signals, standardized at 8 bits, so that each connection requires a cascade of conversions of the PCM signal, compressed to 8 bits, into a differential PCM signal and of the differential PCM signal into the PCM signal, compressed to 8 bits. For the first-mentioned type of conversion an expander must be provided which produces the linear PCM signal to be applied to a differential PCM encoder of the above described type. For the second type of conversion a compressor must be provided at the output of the remote differential PCM decoder for converting the linear PCM signal produced by this decoder into a compressed 8-bit PCM signal.
In a link using a cascade of PCM to differential PCM conversions and the other way around, there is a risk of accumulation of noise produced at each conversion and inherent to the digital character of the processed signals. Particularly, the encoders and decoders for differential PCM signals comprise digital filters; a filter which is identical to the filter of the local decoder, included in the encoder for forming the prediction signal, must be provided in the remote decoder for forming the coded signal; similarly, a filter identical to the syllabic filter included in the encoder forming the quantizing step matching signal, must be provided in the distant decoder; the corresponding filters must of course produce the same signal in response to the same signal. It is a known fact that digital filters, even when the same signal is applied to them, may furnish different signals because of their internal memory. It is clear that if this is also the case for the corresponding filters of the encoders and the distant decoders the differential PCM transmission is not possible without degradation. Moreover, if no precaution is taken in the encoders and decoders to effect the necessary limitation of the number of bits (by means of rounding or truncation), there is the risk of an accumulation of noise resulting from these limiting operations during each PCM differential PCM conversion and inversely. A further risk of accumulation of the noise produced during each conversion resides in the required expanding and compression operation in a link which uses a cascade of conversion. All this may cause an impermissible degradation in the signal in a link using differential PCM signals for the transmission.